The use of gas plasma for processing semiconductor wafers is common in the art. For example, various techniques are described in J. Hollahan and A. Bell, Techniques and Applications of Plasma Chemistry, Ch. 9 (1974).
Semiconductor components are fabricated on a semiconductive substrate or wafer. The material of the wafer is generally silicon. In manufacturing semiconductor devices, a photosensitive polymer, generally referred to as a photoresist, is used. After selective exposure to optical radiation and subsequent chemical development, the photoresist hardens where it has not been removed and protects the underlying wafer from other chemicals. The underlying material on the surface of the silicon wafer, which may be a thin film of aluminum, silicon dioxide, or polysilicon, is then pattern etched with a gas plasma such as carbon tetrafluorine with a small addition of oxygen.
One method of removing photoresist from wafers after it has served its protective function is by using a gas plasma.
In general, the gas plasma used in removing photoresist is oxygen. More particularly, diatomic oxygen is first exposed to an electric field which transforms some of the diatomic oxygen into an oxygen plasma that contains monoatomic oxygen, generally referred to as atomic oxygen. Atomic oxygen is capable of reacting with the photoresist by breaking its polymer chains such that the photoresist is removed from the semiconductor wafer by the combined action of the atomic oxygen and the molecular oxygen. The resultant by-products include gases such as H.sub.2 O, CO and CO.sub.2.
Prior art plasma reactors for removing photoresist, an example of which is shown in FIG. 2A, consist of a cylindrical quartz reactor. A plurality of semiconductor wafers, each of which has a layer of photoresist on its surfaces, are positioned within the reactor. Metal electrodes are positioned around the reactor, one of which is connected to a radio-frequency (RF) generator operating at 13.56 MHz or harmonics of that frequency and the other is connected to ground. The quartz reactor also includes a gas input port and an exhaust port.
Other prior art plasma reactors, not shown, include single-chamber reactor that has an electrode within the chamber, as best exemplified in U.S. Pat. No. 4,230,515. In addition, prior art reactors include double-chamber reactor in which the plasma is generated in one chamber and the work such as photoresist removal is performed in a second chamber. The plasma may be transported between the two chambers either through a narrow channel or through narrow tubes. The primary disadvantage of the double-chamber reactor is the likelihood of plasma degeneration before it could perform the removal of the photoresist, that is, atomic oxygen tends to recombine to diatomic oxygen on the walls of the channel or tubes.
A common occurrence in prior art reactors is the generation of discharges or arcing between the plasma and nearby metallic parts which are at electrical ground. As shown in FIG. 2B, the plasma reactor walls are equivalent, electrically, to two capacitors. The plasma generated within the reactor may be depicted as a resistor. The region within the plasma adjacent to the walls acts as two diodes whose forward direction points into the plasma. The resistance of the plasma is small compared to the resistance of the back biased diode. If, for example, the RF voltage connected to the first electrode is in the order of .+-.1,000 volts, approximately 450 volts are absorbed by each of the quartz barrel walls. This is due to the inherent property of quartz, which has the characteristics of a dielectric. This leaves approximately 100 volts across the plasma, most of which is across the back biased diode. When the RF voltage is at peak positive, the interior of the plasma is approximately +550 volts. Similarly, when the RF voltage is at peak negative, the interior of the plasma is approximately -450 volts. Thus, the voltage between the plasma and the ground is always roughly .+-.500 volts. This high voltage enhances the likelihood of arcing between the plasma and any grounded parts. For example, metal fittings on various parts which are connected to the reactor may be the targets of such arcing, resulting in the overheating and corrosion of those parts. In particular, the arcing may go to the fittings which connect the exhaust manifold to the pump. This arcing limits the amount of power that can be put into the plasma, and consequently, the rate at which the chemical reaction can take place and the capacity of the reactor.